Nowadays, flash memory is widely used in nonvolatile semi con doctor devices. For the purpose of replacing the flash memory, various memory cells are being developed. Particularly known are variable resistance memory cells that store the information of logic 0 and logic 1 using a variable resistance element according to the resistance state thereof.
Variable resistance elements include, for instance, a Re-RAM (Resistive-Random Access Memory) using a metal oxide, sod an STT-RAM (Spin-Transfer Torque Random Access Memory) that performs spin, transfer torque writing using an MTJ (Magnetic Tunnel Junction) element.
The variable resistance element of a ReRAM is initially in a insulated state after being manufactured, and a predetermined voltage (i.e., forming voltage) must be applied to the manufactured variable resistance element to form a filament path inside therein so as to be able to switch between two states: high resistance and low resistance states. This process of forming a filament path is called “forming.” After the forming, the high and low resistance states can be stably switched in response to application of a write voltage different from the forming voltage, achieving a stable memory operation.
Patent Literature 1 discloses a semiconductor memory device comprising a ReRAM as a variable resistance element. A forming is performed on the semiconductor memory device before shipped from tire factory. After the forming, product-specific information for each product (for instance redundancy relieving information if redundancy has been relieved, defect block address information, and product information such as a product number) is acquired as initialization data at a test stage.
Patent Literature 1:
Japanese Patent Kokai Publication No. JP-P2012-79367A, which corresponds to US Patent Application Publication No. US2012/081946A1 and U.S. Pat. No. 8,411,488B2.